How do I change the value of the "To:" text field on the Message actuator?
You need logic set up like in the image below on each of your markers.


It is just an Always sensor, set to True level triggering; and a Near sensor. Those two are plugged in to the python controller, which will activate the message actuator. The property in the Near sensor is not needed, that is only to limit what objects get the message.

The script will send a message to every object that is near (3 units or closer) the marker.

import bge

cont = bge.logic.getCurrentController()

list = cont.sensors['Near'].hitObjectList

if cont.sensors['Near'].triggered:
    message = cont.actuators['Message']
    for obj in list:
        message.propName = obj.name
        cont.activate(message)



The script gets all the objects that are near the marker and stores it as the list variable (line 5).
The if statement works like this, when anything is near the marker, the near sensor is triggered, thus it is True.
It then loops through the list of objects that are near, and sets the message actuator's "To:" field to one of the objects. (message.propName = obj.name)
The last line still part of the loop, activates the actuator - sends the message.